Various approaches in regenerative medicine that are aimed at replacing lost functions of organs are limited by a lack of sources of sufficient cells. Thus, significant research has been directed towards exploring human embryonic stem (hES) cells as a potential cell source. Human embryonic stem (hES) cells are capable of indefinite self-renewal and are pluripotent; that is, these cells are able to differentiate into practically every type of cell found in the organism from which they are derived (2,3). As such, hES cells may provide a supply of different cell types for use in a variety of research and medical purposes.
One such use is in the treatment of renal disease and loss of renal function. The increasing prevalence of type II diabetes has led to a growing incidence of end-stage renal disease. This increased prevalence combined with the limited alternatives for treatment has made kidney disease a major world-wide health problem (2,4).
The major functions of the kidney include the elimination of metabolic waste products and the regulation of water, electrolyte and acid-base balance. These are mainly achieved by (i) a filtration process that takes place in the glomeruli and leads to the formation of a primary filtrate volume of 180 l/day, and (ii) a reabsorption process in the subsequent tubular system (5-7). Upon filtration, most of the water and the dissolved substances return through transport over the proximal tubulus epithelium, back to the capillary system with or without reduced reabsorption of the metabolic waste products (5-7). The massive water reabsorption is enabled by a specific water-transporting channel protein named aquaporin-1 (AQP-1), which is localized on the cell membranes of epithelial cells, lining the renal proximal tubulus structures as a monolayer (8-10).
Artificial devices have been designed to mimic renal function. In such devices a size-selective filtration process normally performed by the glomeruli can be performed by polymer membranes that are assembled as hollow fibers in cartridges, and devices for hemodialysis or hemofiltration have been proven successful in clinical applications. In contrast, development of methods to mimic the renal reabsorption function seems to be highly dependent on the specific functionalities of renal cells (11-12).